Many electronic systems employ programmable gain amplifiers as a basic analog building block. An example of a conventional programmable gain amplifier (900) is illustrated in FIG. 9. Programmable gain amplifier 900 includes an operational amplifier (AMP9), three resistors (R91-R93), and two switches (S91-S92). The operational amplifier includes a non-inverting input terminal (INP), an inverting input terminal (INM), and an output terminal (AOUT). Switch S91 is connected between AOUT and node N91. Switch S92 is connected between AOUT and node N92. Resistor R91 is connected between node N91 and INM. Resistor R92 is connected between node N92 and INM. Resistor R93 is connected between INM and a circuit ground (GND).
In operation, operational amplifier AMP9 is configured to receive an input signal (e.g., IN) at the non-inverting input (INP), and provide an output signal (e.g. OUT) at the output terminal (AOUT) that is related to the input signal according to a gain scaling factor (X). The gain scaling factor (X) is determined by the impedance in the feedback path between the output (OUT) and the inverting input (INM).
Switches S91 and S92 are arranged to change the selection of the feedback components (resistors R91 and R92) to adjust the gain scaling factor (X). The selected feedback component is resistor R91 when switch S91 is closed, while the selected feedback component is resistor R92 when switch S92 is closed. The relationship between the input signal (IN) and the output signal (OUT) is determined by the feedback components and the switches as follows: OUT=IN*(1+X), where X is determined by the feedback components. In one example, X=R91/R93 when switch S1 is closed and switch S2 is open. In another example, X=R92/R93 when switch S2 is closed and switch S1 is open.